1. Field of the Invention
The present invention relates to a memory module installed in a memory extension slot of a computer, and more specifically to a memory module using a DRAM (dynamic random access memory) which requires refreshing of memory contents at or within predetermined time intervals. The invention is also directed to a method of refreshing the memory module.
2. Description of the Related Art
An element typically used for main storage of a computer is a DRAM having a simple internal structure and readily increasing a storage capacity. For the reason of cost reduction and appropriate specification, a general computer has only a DRAM, having a capacity sufficient for allowing operation of typical software, as a main memory unit. When the user requires a larger storage capacity, a separately available memory module having a DRAM is inserted into a memory extension slot of the computer. This realizes inexpensive and easy extension of the storage area.
The DRAM used for data storage in the computer stores information by accumulated charges in a capacitor. A refreshing process for re-writing the memory contents at predetermined intervals is thus required for effective data storage and retention. When the computer executes such refreshing process, however, large loading applied onto the computer undesirably decreases the speed of data processing of the computer. An improved DRAM recently proposed has a self-refresh mode for executing the refreshing process. The DRAM having the self-refresh mode does not require output of an instruction signal or an address signal for refreshing (hereinafter referred to as refresh signal) from the computer to the DRAM, and works as if the memory module is constructed as an SRAM (static random access memory). This effectively reduces the loading applied onto a CPU of the computer with the DRAM.
The DRAM having the self-refresh mode includes a specific circuit for executing internal refreshing. This specific circuit, however, has a complicated structure and occupies a large space in a restricted area of a semi-conductor chip. The self-refresh mode is accordingly applied to DRAMs having a relatively small storage capacity. The computer is generally designed to output an instruction signal for refreshing, thus being applicable to a memory module which has a DRAM without the self-refresh mode and is inserted into an extension slot of the computer.
When the computer has only one memory extension slot, a memory module inserted into the extension slot should have a large capacity DRAM for significant increase of the main storage area. Since the large capacity DRAM uses substantially all the area of the semi-conductor chip for the data storage, it is extremely difficult to mount a complicated circuit for generating signals for self-refreshing on the DRAM.
A book-sized personal computer is generally designed to save power consumption, and thereby has a sleep mode in which a clock frequency is lowered or output of signals to an external device is interrupted when a keyboard or a mouse has not been used for a predetermined time period. When a memory module having a large capacity DRAM without the self-refresh mode is inserted into an extension slot of a computer with the sleep mode, output of the refresh signals from the computer to the memory module is interrupted during the sleep mode, which prevents the memory contents being refreshed and undesirably erases the memory contents.